Radio frequency (RF) signals are typically routed between RF circuits in an RF communications system. The integrity of the RF signals may depend on transmission line characteristics, such as impedance, of RF signal paths between the RF circuits. FIG. 1 illustrates a typical RF signal path between two circuits. Specifically, FIG. 1 shows an RF signal source 10 feeding a load 12 through an RF transmission line 14, according to the prior art. The RF signal source 10 provides an RF output signal RFOUT to the RF transmission line 14, which conveys the RF output signal RFOUT along a transmission line length to provide an RF load signal RFLD to the load 12. With perfect impedance matching between an RF signal source output and the RF transmission line 14, and between the RF transmission line 14 and the load 12, power transfer from the RF signal source 10 to the load 12 is maximized without reflections of the RF power.
If there is an impedance mismatch between the RF transmission line 14 and the load 12, some or all of the power may be reflected back toward the RF signal source 10, thereby reducing the quantity of power transferred to the load 12. The power conveyed along the transmission line length toward the load 12 is forward power PF and the power conveyed along the transmission line length reflected back from the load 12 is reverse power PR. The power delivered to the load 12 is delivered power PD, which is the difference between the forward power PF and the reverse power PR. Some applications may require measurements of the delivered power PD, the forward power PF, the reverse power PR, or any combination thereof.
FIG. 2 shows an RF power meter 16 coupled between the RF signal source 10 and the load 12 illustrated in FIG. 1. The RF power meter 16 may measure the delivered power PD, the forward power PF, the reverse power PR, or any combination thereof. The RF power meter 16 may be installed temporarily to take power measurements during manufacturing, testing, field service, or the like, or may be installed permanently as part of an RF communications system to take ongoing power measurements. The RF power meter 16 may introduce insertion loss, which reduces efficiency and may impact the accuracy of power measurements. Additionally, the RF power meter 16 may operate over a narrow frequency range and may have narrow impedance requirements for proper operation. Some RF power meters 16 may use directional couplers to differentiate between forward power PF and reverse power PR; however, directional couplers may be bulky, costly, and have significant insertion losses.
FIG. 3 shows an antenna 18 as the load 12 illustrated in FIG. 2. The RF signal source 10 may be an RF power amplifier, which may provide RF transmit signals to the antenna 18 through the RF power meter 16. Such an arrangement may be used in a portable wireless device, such as a cell phone. In some portable wireless devices, the delivered power PD to the antenna 18 must be measured and controlled to meet transmit power requirements of a communications standard, specific absorption ratio (SAR) requirements, or the like. Additionally, some portable wireless devices support multiple communications standards using different frequency bands and must operate over a wide range of environmental conditions. The antenna 18 in a portable wireless device is typically subjected to changing RF conditions, such as varying proximity to a user's body, to metallic objects, or the like. Such changing RF conditions may change the impedance of the load 12, thereby affecting power measurements. Thus, there is a need for a small and inexpensive RF power measurement circuit that can accurately measure delivered power PD over a wide frequency range, with low insertion loss, over a wide range of environmental conditions, into a load 12 having a changing impedance.